Dihydropyrimidine dehydrogenase (DPD) is the first and rate limiting enzyme in the three step metabolic pathway of the catabolism of thymine and uracil. Among the end products of pyrimidine catabolism is beta- alanine. By virtue of its ability to degrade pyrimidines, DPD is responsible for the metabolism of fluorouracil (5-FU). A number of patients with cancer receiving a therapeutically adjusted dose of 5-FU exhibited unexpected severe toxicity the, among a variety of clinical symptoms, included neutropenia, thrombopenia and neurological damage. A number of cases of 5-FU toxicity have been documented that are related to low DPD activities. The association of high levels of toxicity with a simple decrease in enzyme activity is not particularly surprising since most anti-cancer drugs exhibit narrow therapeutic indices. Marked inter-patient variability with respect to pharmacokinetics for most chemotherapeutic drugs used in cancer treatment can result in adverse toxicity and lack of efficacy. Thus, careful determination of the patient's metabolic capacity to inactivate a specific anti-cancer agent, such as 5-FU can minimize adverse side-effects and maximize efficacy, resulting in a better prognosis for each individual patient. It is noteworthy that patients that are heterozygous for the DPD deficiency can exhibit toxicity to 5-FU. Infants born with a compete deficiency in activity have a condition called thymine uraciluria and can display a number of debilitating phenotypes including convulsions, microcephaly and developmental delay. To determine the molecular basis for the DPD deficiency, the human DPD cDNA and gene were cloned and sequenced. A mutation was found in a cancer patient's DPYD allele that results in a G to A base change at the 5' splice site of exon 14 of the gene. This mutation was also found in the homozygous state in a Dutch patient with thymine-uraciluria and completely lacking DPD activity. A second family of Pakistani origin was found with two brother both of whom were homozygous for the DPYD*2 mutant allele. One brother was affected with the common symptoms while the second brother was asymptomatic indicating that the complete deficiency is not completely penetrant. These findings suggest a possible gene-environment interaction. A genotyping test for the G to A splicing point mutation could be useful in predicting cancer patients prone toxicity upon administration of potentially toxic 5-FU and for genetic screening of heterozygous carries and homozygous deficient subjects.